STRUCTURE. A heart contained in its peri cardial sac is always present, except in the para sitic Entoconcha, while in some genera, as Neritina (periwinkle) and Haliotis (abalone), it, as in the clam, is perforated by the intestine. In a few genera there are two auricles to the heart, but as a rule only one is present. A ventricle is always present. There is but a single kidney (nephriffium). The digestive canal is doubled on itself, the vent opening on one side of the mouth. In certain opisthobranchs the stomach is lined with series of teeth, sometimes sharp and chitinous. In some nudibranch gas tropods (see NUDIBRANCHIA) the intestine has numerous lateral offshoots, or gastro-hepatic branches, which resemble similar structures in the planarian and nematode worms.
The nervous system varies in the number of ganglia, but is usually represented by the 'brain,' a pair of suprawsophageal ganglia, with con necting threads (commissures) passing around the gullet to the infrawsophageal or pedal gan glia, thus forming the esophageal nerve-ring; there are also a pair of buccal ganglia, while the visceral and abdominal ganglia, all connected by commissures, are situated at a varying dis tance from the head. The ears, or `otocysts,' are usually near the pedal ganglia, but are always innervated from the cerebral ganglion, or 'brain.' The animal in certain forms is bisexual or hermaphroditic, in others the sexual glands exist in separate individuals. The eggs are laid in cap sules of various sizes and shapes, usually attached to seaweeds or rocks, or deposited freely in the sand. Land-snails lay their eggs loose under stones or leaves in damp places. The embryo on hatch ing passes through a well-marked metamorphosis, the two more important stages being the trocho sphere and veliger, the latter differing from the trochosphere or top-shaped primitive stage in swimming about by means of a pair of sail-like flaps.
Soon after the shell of a gastropod begins to form, the foot grows larger, the eyes and ten tacles appear, when the young sinks by gravity to the bottom and gradually assumes the snail condition of maturity. The eyes may be absent in those marine forms which actively burrow in the sand, though the single pair of tentacles per sists. In the land-snails there are two pairs of tentacles, the upper and longer pair containing both the eyes and the optic nerve with the olfac tory nerve, which ends in groups of cells.
A distinctive feature in gastropods is the 'odontophore,' an apparatus of muscles bearing the radula or 'lingual ribbon,' a solid flattened ribbon-like or rasp-like plate armed with trans verse rows of sharp siliceous teeth. This rasp is drawn back and forth over a tendon like a pulley. By means of this rasp the land or pond snail cuts slits into leaves, swallowing the pieces, or the marine forms, such as the Sycotypus (see Cox en) or the 'drill,' files a hole into the clam or oyster, so as to get at the flesh within the tightly closed shell of its victim.
Certain forms, as the Murex (q.v.) of the Mediterranean, secrete the Tyrian dye of the an cients, and a similar fluid is secreted by the corn mon Purpura of our coast. This fluid is formed in a peculiar `adrectal gland' situated at the side of the rectum. It is colorless, but turns purple on exposure to the air.
The shell of different gastropods varies greatly in shape. In the limpets (Patella) it is low and conical; in most of the species it is spiral, made up of whorls. The greater number of shells are `dextral,' i.e. the spire turns to the right; in a few cases they are sinistral or turn to the left. Over 22,000 species are known, of which about 7000 species are fossil; there are about 6500 species of land-snails alone.